The long-range goal of this research program is to understand structure/function relationships that underlie information processing in the lateral geniculate nucleus (LGN) and visual cortex, and the interrelationships between the two brain regions. Experiments proposed here will address questions about the cell types which exist in the LGN, the mechanisms which affect their signal processing, and the cells' connections in visual cortex. The impetus for the experiments is the finding of a new type of X- relay cell (term lagged, or X cells) in the LGN A-laminae, whose functional and structural properties differ from other X-relay cells (termed non-lagged, or X - cells) and which appear to reflect intrageniculate inhibitory mechanisms related to cell structure. The present goal is to learn more about the physiological and morphological properties of these two geniculate X-cell groups and to identify the cortical cells which receive from them. The properties and projections of Y-cells, which probably also contain lagged and non-lagged groups, will be examined as well. Specific aims are as follows: (1) To characterize further the inhibitory influences on lagged and non-lagged LGN cells. (2) To examine binocular interactions in these cells and the influence of these interactions on the cells' visual response properties. (3) To reveal the role of the brainstem reticular formation in modulating the activity of lagged and non-lagged relay cells, and the potential involvement of different types of interneurons in this modulation. (4) To reveal the relationships between cells' morphologies and the physiological properties examined in aims 1- 3. (5) To reveal the role of GABAergic inhibition in generating the visual response properties of lagged LGN cells. (6) To determine the cortical connections of lagged and non-lagged cells in layer 4 of area 17, and (7) to visualize the morphologies of the cortical cells which these afferents contact. Standard extracellular recording, visual and electrical stimulation methods and intracellular staining will be used to accomplish these aims. Standard pharmacological methods also will be used to accomplish aim #5. The results obtained will reveal important insight into structural and functional mechanisms underlying visual information processing in LGN and visual cortex.